The main objective of this proposal is to study the first specific enzyme in the cholesterol biosynthetic pathway in the liver, squalene synthetase, in order to understand the mechanisms of its regulation. This objective is now attainable since we recently isolated, purified and obtained partial protein sequences of the rat liver squalene synthetase protein as well as cDNA clones for both the rat and the human enzymes. This, in turn, will enable us to clone and study the regulation of the enzyme. The mechanisms of regulation of rat hepatic squalene synthetase at the transcription, translation and protein level will be examined under wide variations of cholesterogenesis. The contribution of the variations of the activity of this enzyme to the diversion of common intermediates, specifically trans-farnesyl pyrophosphate, to sterol and nonsterol products (the flux diversion hypothesis) will be examined. Examination and comparison of 5'flanking regions of the squalene synthetase gene to other known regulatory enzymes in this pathway may explain the complex regulation of the mevalonate branched pathway. Cloning and the understanding of the regulation of squalene synthetase, the most specific enzyme in the cholesterol biosynthetic pathway, may allow the development of compounds that will interact with, and change the activity of this enzyme and therefore, may serve as cholesterol lowering drugs.